Durability

Large surface area solids that accelerate the rate of attaining equilibirum; they are themselves chemically unchanged by the reaction and therefore do not affect the final equilibrium yields

Those unique sites on the surface where the catalytic event takes place. They are usually the steps, edges and kinks of the flat unreactive terraces, and often represent only a few percent of the total surface atoms. Interestingly, catalysis is chemistry at imperfections and discontinuities.

Rate of reaction per unit area of catalyst under standard reactant concentrations. Activity is sometimes defined empirically by the temperature needed for a reaction to reach an arbitrary rate of conversion.

Number of product molecules formed per second per active site (10~2 to 102s~1 for synthetic catalysts, compared with 103 to 107 for enzymes).

Rate dependent on particle size and crystal face/rate invariant to size and face.

The fraction of reacted molecules converted to a desired product

How long the catalyst remains active the accelerating effects of small amounts of materials that themselves were chemically unchanged by the reaction. He is sometimes erroneously quoted as suggesting an almost occult catalytic force, whereas in fact he clearly stated the new 'force' is 'a manifestation of the electrochemical affinities of matter' Michael Faraday had a clear concept of catalysis in 1834 even before Berzelius' presentation of the term to the Swedish Academy of Sciences. It is tempting to associate the discovery of catalysts with the alchemists vain search for the philosopher's stone to transmute base metals into gold; in some respects the traces of 'catalyst stone', which transform 'useless' raw materials into useful products, are more valuable than a stone for making ornamental gold. The appellation 'black art' still lingers, perhaps with some justification, since most industrial catalysts are still formulated by recipes that are incompletely understood and retain an aura of mystery.

The burgeoning nineteenth century chemical industry quickly appreciated the utility and exploitability of catalysts and the lead chamber and contact processes developed rapidly, as did the hydrogenated hardening of vegetable oils and the synthesis of indigo. Perhaps the most important catalyst ever developed, because it averted world famine, was that for the high pressure synthesis of ammonia by Fritz Haber in 1909, and its commercialization by Badische Aniline and Sodafabrik. The technical development of the iron catalyst, which remains largely unchanged, exemplifies the rewards of painstaking research, a full appreciation of thermodynamics and the courage to build large scale industrial plants operating at the then unprecedented pressure of 300 atmos pheres at 450°C. The process is a tour de force of chemical engineering linked with outstanding science.

The molecular understanding of catalysis really begins in the twentieth century, as have the majority of the catalysts and processes now in use. This is especially true of the petroleum and petrochemical industry, which in the 1960s and 1970s had annual growth rates of 15%. The outstanding development in the interwar years was the Houdry process for the catalytic cracking of heavy petroleum fractions into volatile gasoline components (C2-C13). The immediate postwar period saw developments such as: (1) platforming catalysts for upgrading gasoline research octane numbers (RONs) by isomerization of straight into branched alkanes and the dehydrogenation of cyclohexanes to aromatics; and (2) steam reforming of methane, naphtha and fuel oil feedstocks into synthesis gas, a carbon monoxide/hydrogen mixture important for ammonia and methane production. The recent development of vehicle exhaust catalytic converters based on rhodium-promoted platinum-alumina for converting carbon monoxide, nitrogen oxides and unburnt hydrocarbons into harmless products is a remarkable achievement for catalysis science, especially in view of the varying gas compositions and temperatures that have to be sustained.

The realization by Langmuir that catalysis involved short-range chemical bonds, which dictated that reactions occurred in surface monolayers, and Taylor's suggestion that catalysis occurred only at unique active sites, together with the discovery of the importance of atomic clusters with specific geometric arrangements by Balandin, were seminal to the growing

Table 2 Important stages and concepts in heterogeneous catalysis

Date

Event

The beginning! Life originates on the surfaces of clay minerals Prehistory Enzymes for making wine and vinegar, sterile silver

1834/1835 Berzelius coins the term catalysis

Faraday appreciates the occurrence of catalysis

1830-80 Industrial: H2SO4 by lead chamber, contact catalysts; synthetic dyes by mercury catalysis; hardening of oils 1900 Start of modern kinetics by Bodenstein and

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.